Bourdon Tube-Type Detectors:

The bourdon tube pressure instrument is one of the oldest pressure sensing instruments in use present days.  The bourdon tube that was display in below figure consists of a thin-walled tube which is flattened diametrically on opposite sides to produce a cross-sectional area elliptical in shape, having two long flat sides and two short round sides. A tube is bent lengthwise into an arc of a circle of 270 to 300 degrees. The Pressure applied to the inside of the tube causes distention of the flat sections and tends to restore its original round cross-section. This modification within cross-section causes a tube to straighten slightly.

Figure: Bourdon Tube

Because the tube is permanently fastened at one end, a tip of the tube traces a curve which is the result of the modification in angular position along with respect to the center.  Inside limits, the movement of the tip of the tube could then be used to position a pointer or to develop an equivalent electrical signal (that is discussed later in the text) to denotes the value of the applied internal pressure.